In the fabrication of semiconductor-based devices, e.g., integrated circuits or flat panel displays, layers of materials may alternately be deposited onto and etched from a substrate surface. As is well known in the art, the etching of the deposited layers may be accomplished by a variety of techniques, including plasma-enhanced etching. In plasma-enhanced etching, the actual etching typically takes place inside a plasma processing chamber. To form the desired pattern on the substrate surface, an appropriate mask (e.g., photoresist) is typically provided. A plasma is then formed from a suitable etchant source gas, or mixture of gases, to etch areas that are unprotected by the mask, leaving behind the desired pattern.
To facilitate discussion, FIG. 1A depicts a simplified plasma processing apparatus 100 suitable for fabrication of semiconductor based devices. Plasma processing apparatus 100 includes a plasma processing chamber 102 in which process parameters are tightly controlled to maintain consistent etch results. Process parameters governing etch chemistry may include plasma composition, plasma excitation, power supplied to the plasma, gas flow, plasma chamber temperature, and plasma chamber pressure. Since the etch tolerance (and resulting semiconductor-based device performance) is highly sensitive to such process parameters, accurate and repeatable control of the process parameters is required.
One or more manometers are conventionally used in order to facilitate pressure control within plasma pressure chamber 102. For example, the plasma processing apparatus 100 may include a manometer tree 104. The manometer tree 104 includes two manometers 106 and 108 which are responsible for sensing pressure within plasma processing chamber 102. Typically, the manometers 106 and 108 operate in different ranges. While one manometer may have a broad range of operation and low resolution, the second manometer generally has a smaller range of operation and higher resolution sensitivity based on the process conditions typically encountered in the plasma processing chamber 102. The manometers 106 and 108 electrically communicate with a computer 110 to facilitate monitoring and control. The computer 110 also couples with a monitor 112 to provide a suitable user interface.
In recent years, continuing miniaturization of modem semiconductor devices to critical dimensions below 0.25 microns has necessitated substantially lower process pressures of some etch processes and thus finer pressure control within the plasma pressure chamber 102. Currently, etch processes must often be repeatedly performed within a tolerance on the order of one tenth of a milliTorr (mTorr). As operating pressure outside this range may compromise performance of the semiconductor-based device, accurate and repeatable control of the plasma chamber 102 pressure within this tolerance is required.
FIG. 1B illustrates a conventional manometer 120 which may represent either manometer 106 or 108 of manometer tree 104. Manometer 120 is in gaseous communication with the plasma processing chamber 102. Manometer 120 is coupled either directly or indirectly to the plasma processing chamber 102. Inlet area 122 may either represent the plasma processing chamber 102 or, in the case of indirect coupling, may represent a portion of the manometer tree 104 which is in gaseous communication with the plasma processing chamber 102. An ambient area 123 lies within the manometer 120 and operates at similar environmental conditions as inlet area 122.
A pressure sensitive diaphragm 124 is provided within the manometer 120. The pressure sensitive diaphragm 124 allows sensing of the pressure within plasma processing chamber 102. The pressure sensitive diaphragm 124 is flexible in the direction shown by arrow A in FIG. 1B and thus allows passive response to pressure variation within ambient area 123. Commonly, the pressure sensitive diaphragm 124 acts in conjunction with rigid plate 130 to form a pair of corresponding capacitive plates. Optionally, a capacitive material may be disposed between the two plates in a sealed area 128. Thus, a capacitor is formed from the pressure sensitive diaphragm 124 and the rigid plate 130 whose capacitance is dependent on the extent of flexing of the pressure sensitive diaphragm 124. The capacitor within manometer 120 allows sensing of the pressure within the ambient area 123 as well as outputting an electrical output corresponding to the measured pressure. As shown in FIG. 1A, an electrical communication link 114 from the manometers 106 and 108 typically leads to a monitor 112 or computer 110 for user interface and control purposes.
It has been observed, however, that the performance of conventional high-resolution manometers tends to operationally degrade significantly over the operational lifetime of the manometers. For example, manometers responsible for pressure detection in the range of 0-20 mTorr range have been found to significantly degrade, particularly during early implementation of the manometer. One such degradation is referred to as "drift". Drift may be loosely described as a consistent difference between manometer output and the actual pressure in the ambient area 123. For example, unacceptable zero point drift, or non-zero readings of the manometer 120 for zero pressure in the plasma processing chamber 102, can commonly occur. As a result of this transient and permanent drift, operating pressures within plasma processing chamber 102 frequently exist outside allowable tolerances over the service life of the manometer. This seriously compromises fabrication of semiconductor-based devices.
In view of the foregoing, an improved manometer suitable for use in a plasma processing environment is required.